(1) Field of the Invention
This invention relates to polarizers for converting a linearly polarized microwave signal to an elliptically polarized microwave signal and vice versa.
(2) Prior Art
The prior art teaches a variety of ways to convert a linearly polarized microwave signal to a circularly polarized microwave signal and vice versa. For example, the transformation between a linear and a circular polarization can be accomplished by a septum polarizer. A septum polarizer usually is a threeport waveguide device where the number of ports refers to the physical ports of the devices described hereinafter. It may be formed from circular waveguides, but more typically is formed by two rectangular waveguides that have a common wide or H-plane walls. The two rectangular waveguides are transformed by a sloping septum into a single square waveguide. Various prior art septum polarizer designs are illustrated and described in U.S. Pat. No. 3,958,193 issued May 18, 1976 to James V. Rootsey, assigned to Aeronutronic Ford Corporation now Ford Aerospace and Communications Corporation, the assignee of the present invention.
In a septum polarizer, a linearly polarized transverse electric field microwave signal is converted, through the action of the septum, into a circularly polarized (CP) microwave signal and vice versa. The linearly polarized signal is introduced into one of the two rectangular waveguide ports and produces in the square waveguide port a microwave signal having either right-hand circular polarization (RHCP) or left-hand circular polarization (LHCP). Whether (RHCP) or (LHCP) is produced depends upon which of the two rectangular waveguide ports is excited. It is possible and in some applications very desirable to introduce simultaneously in both of the rectangular waveguide ports linearly polarized signals to produce in the square waveguide port both RHCP and LHCP signals or vice versa. The two linearly or circularly polarized signals may constitute separate information channels. If the RHCP and LHCP signals co-existing in the square waveguide port have perfect circular polarization characteristics, they are completely isolated from one another and there is no interference between them.
A perfect CP signal has a rotating electric field that can be regarded as the vector resultant of two orthogonal components E.sub.x and E.sub.y having sinusoidally varying magnitudes that are exactly equal in amplitude but 90.degree. out of phase with one another. The closer simultaneously existing RHCP and LHCP signals come to the perfect CP signal, the greater is the isolation between them. The axial ratio AR is the ratio of E.sub.x to E.sub.y and is an indication of the degree to which a CP signal has departed from the ideal. In dB, the axial ratio AR is equal to 20 log E.sub.x /E.sub.y. Perfect CP signals have an AR of 0 dB.
The problem associated with prior art polarizers is their inability to provide low axial ratios over a moderately wide frequency band and also to provide a low voltage standing wave ratio (VSWR) over such band. In order to convert a perfectly linearly polarized signal to a perfectly CP signal or vice versa, the polarizer must produce exactly 90.degree. phase shift between one of the orthogonal components of the CP signal electric field and the linear electric field in the rectangular waveguide port. Many prior art designs provide a phase-shift-angle vs. frequency function that has no inflection point in its slope. In other words, the phase shift angle, as a function of frequency over the useful frequency range of the polarizer, has a rate of change or slope that remains either positive or negative (whether the slope is positive or negative depends upon the conditions selected as a reference.). The phase angle deviations from 90.degree. produce axial ratio increases of about 0.15 dB/degree difference from 90.degree.. Prior art designs which do have an inflection in the phase shift angle vs. frequency function are not readily compatible with all antenna types and have limited flexibility in selecting the slope of the function around the point of inflection. In particular, there are applications for which the prior art does not provide a sufficiently broad frequency band with a sufficiently low axial ratio. These are some of the problems this invention overcomes.